Generally, a semiconductor substrate, or integrated circuit, is mounted on a printed circuit board or the like and the accepted method for connecting the substrate to external circuits is to use standard wire bond technology. However, when a semiconductor substrate having a relatively large array of electrical components or devices formed thereon is to be connected, standard wire bond techniques can become very difficult. For example, if a relatively large array (greater than, for example, 10,000 or 100.times.100) of light emitting devices, or diodes, (LEDs) is formed on a substrate with a pitch (center-to-center separation) of P, then bond pads on the perimeter of the substrate will have a 2P pitch. This is true because every other row and every other column goes to an opposite edge of the perimeter to increase the distance between bond pads as much as possible.
At the present time wire bond interconnects from bond pads having a pitch of 4.8 milli-inches is the best that is feasible. Thus, in the array mentioned above of 100.times.100 LEDs the bond pads on the perimeter of the substrate would have a minimum pitch of 4.8 milli-inches, with 50 bond pads situated along each edge of the perimeter. As more devices are included in the array, more bond pads are required and the perimeter size to accommodate the additional bond pads increases at an even greater rate. That is, since the minimum pitch of the bonding pads is 4.8 milli-inches, the pitch of the devices in the array can be as large as 2.4 milli-inches, or approximately 61 microns, without effecting the size of the substrate. Thus, even if the devices can be fabricated smaller than 61 microns, the minimum pitch of the bonding pads will not allow the perimeter of the substrate to be made any smaller. It can quickly be seen that the size of the substrate is severely limited by the limitations of the wire bonding technology.
When the array of LEDs is fabricated with organic materials, several additional problems are prevalent. A two-dimensional organic LED array for image manifestation apparatus applications is composed of a plurality of organic LEDS (one or more of which form a pixel) arranged in rows and columns. Each individual organic LED in the array is generally constructed with a light transmissive first electrode, an organic electroluminescent medium deposited on the first electrode, and a metallic electrode on top of the organic electroluminescent medium. The electrodes of the LEDs are connected to form a two-dimensional X-Y addressing pattern. In practice, the X-Y addressing pattern is achieved by patterning the light transmissive electrodes in an X direction and patterning the metallic electrodes in a Y direction (or vice versa if desired), with the X and Y directions being perpendicular to each other. The patterning of the electrodes is usually accomplished by either shadow mask or etching techniques. Due to the technical limits of shadow masks, etching processes are generally being utilized for high density information displays, which have pixel pitches less then 0.1 mm.
Depending on the medium used in the etching processes, the etching technique can be divided into two categories: wet and dry. While wet etching is generally performed in an acidic liquid medium, dry etching is usually done in a plasma atmosphere.
The metallic electrodes used for cathode contacts in organic LEDs usually contain a stable metal and a highly reactive metal with a work function less then 4 eV. The presence of the highly reactive metal in the metallic electrode makes acid-based wet etching undesirable. However, the dry etching processes is also problematic because of the high temperature (&gt;200.degree. C.) and reactive ion atmosphere required in the process, which may affect the integrity of the organic materials as well as the active metal containing metallic electrodes in a two-dimensional organic LED array.
To overcome the etching dilemma, a shadow wall method to fabricate the two-dimensional array has been disclosed by Tang in a U.S. Pat. No. 5,294,870, issued Mar. 15, 1994 and entitled "Organic Electroluminescent Multicolor Image Display Device". The shadow wall method includes: patterning the transparent electrode first; building dielectric walls that are orthogonal to the transparent electrodes, capable of shadowing an adjacent pixel area, and with a height exceeding the thickness of the organic medium; depositing an organic electroluminescent medium; and depositing the cathode metals from an angle of 15.degree. to 45.degree. with respect to the deposition surface. Since the height of the dielectric walls exceeds the thickness of the organic medium, isolated parallel metal stripes are formed. Thus, a X-Y addressable array is achieved without the need of metal etching. Though this method seems to be viable for metal patterning, it is limited to certain pitch dimensions, and potentially could introduce defects in pixels in the array.
Also, most organic LEDs are highly susceptible to ambient conditions, especially moisture. Because of the impact of moisture on the performance of organic LEDs, the LEDs must be in a total hermetic enclosure.
Thus, there is a need for interconnect and packaging structures and techniques in which organic LEDs can be conveniently incorporated.
There is also a need for interconnect and packaging structures and techniques which can substantially reduce the limitation on size of the package.
Accordingly, it would be highly advantageous to provide a new LED array/package and method of manufacturing which overcame these problems.
It is a purpose of the present invention to provide integrated electro-optical packages in which organic LEDs can be conveniently incorporated.
It is also a purpose of this invention to provide a package incorporating a novel method of fabricating a two-dimensional organic LED array for high density information image manifestation apparatus applications.
It is another purpose of this invention to provide an organic LED device package on which metal etching can be performed without effecting the LEDs.
It is still another purpose of this invention to provide a passivated two-dimensional organic LED array and package for high density information image manifestation apparatus applications with improved reliability.
It is another purpose of the present invention to provide integrated electro-optical packages with organic LEDs which are not limited in size by the electrical connections.
It is yet another purpose of the present invention to provide integrated electro-optical packages containing arrays of organic LEDs which are hermetically sealed from ambient conditions.
It is a further purpose of the present invention to provide methods of fabricating integrated electro-optical packages incorporating organic LEDs which do not damage the LEDs during fabrication.